Hot body, cold body, working body diagram or view of the Carnot engine.

In thermodynamics, a cold body is the body of lower temperature, in the structure of a heat engine, as compared to another, called the hot body, in which it is elevated. In equation form:

TA (hot body) > TB (cold body)

The cold body is typically referred to as the condenser or refrigerator, typically a stream of cool water, which is sprayed onto the working body during the contraction phase, which serves as a heat sink, as contrasted with a heat source (the boiler), readily able to take up or absorb heat from the working body of expanding and contracting substance (typically water) in side of the piston and cylinder.

History
The definition of the cold body was defined by French physicist Sadi Carnot in 1824 in his description of the production of motive power in steam engines or heat engines, in which he utilized the logic of the cycle or Carnot cycle. To quote:

“The production of motive power [work] in the steam engine is due the transportation of caloric [heat] from a warm body to a cold body, i.e. to its re-establishment of equilibrium—an equilibrium considered as destroyed by any cause whatever, by chemical action, such as combustion, or by any other.”

This terminology was taken up French engineer Emile Clapeyron in 1834 who graphically described the Carnot cycle utilizing the hot body / cold body terminology. [2]

Human thermodynamics
In studies of human thermodynamic systems, the location of the cold body is an intricate subject of study. The basic model of any generic social system is that the cool night sky acts as the cold body. In studies of small number human molecule interactions, a cold body can be distinguished by a facet of perceptual physical or mental anti-beauty qualities. When a relationship grows "cold" for example, it signifies a point in time when the relationship is said to stop "working", as in "its not working between us anymore", a state signified by a volume contraction, in that a person in this state has no energy or drive to get out and be productive goal-driven person in life. In another sense, cold people are said to absorb energy from others. In these types of studies, the determination of where the three different bodies (hot, cold, and working) are becomes a very complicated subject. [3]

References
1. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power (excerpt)” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
2. Clapeyron, Emile. (1834). “Memoir on the Motive Power of Heat”, Journal de l’Ecole Polytechnique. XIV, 153 (and Poggendorff's Annalender Physick, LIX, [1843] 446, 566).
3. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (Google books). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (Google books). Morrisville, NC: LuLu.